The wrong way

First of all, I have to say that the usual way of reading the jetting by the colour of the sparkplug insulator nose is wrong. The colour of the insulator is affected by the jetting and the heat range of the plug. So an improper heat range sparkplug with correct jetting or an correct sparkplug with wrong jetting will produce an colour that will misslead you in reading the jetting. Let's make some examples;

1. Your jetting is ok, and your sparkplug is of an relatively hot heatrange. The colour of the insulator nose will result of a bright gray - white colour. It will make you think that your jetting is lean and make you enrich the jetting. The result is loss of power or/and fouled sparkplug..

2. Your jetting is lean and your sparkplug heatrange is cold.The colour of the insulator nose will result brown, making you think that everything is ok. But you may end up overheating.

The proper way

Some facts about the sparkplug:

The sparkplug insulator nose must have the self-cleaning temperature. The self-cleaning temperature is determined by the heatrange of the sparkplug. This is the temperature at which the carbon deposits that form on the nose are burnt away. Too cold sparkplug heatrange won't burn away the constantly forming carbon deposits, while too hot heatrange may not remove the heat sufficiently and cause electrodes meltdown and other associated overheat problems.

The mixture ring

This sparkplug is an NGK B9EGV it was used for about 3 races on my racing ZIP SP. The insulator nose is quite clean, that tells us that the heatrange was properly selected. It recieved an plugchop, so we can clearly see what we should look for; the mixture ring. The mixture ring forms on the bottom of the insulator nose. It tells us how the engine was jetted.

From the mixture ring on the picture above, we can say that jetting was close to perfect.

If the jetting would be too rich, the mixture ring would thicken, scale up towards the nose tip.

If the jetting would be too lean, the mixture ring would thin down towards the bottom, when the ring would become really thin or disappear, then we could have big problems.

Yes, it's simple as that. However a much bigger problem is to properly see the mixture ring without destroying the sparkplug each time we check the jetting. An good eye with good sunlight may be enough to see inside, or else lens with additional light should be used.

Guidelines

On a daily used engine there is no need to sharply tune it, so jetting is usually on the rich side to prevent it from having big problems, but if the engine is raced, every bit of power should be extracted from it, so jetting should be adapted to weather and track. These are some guidelines to set the jetting:

Cold, dry (high or no clouds) - air pressure is high; bigger jets.

Rainy, fog (low clouds, fog) - air pressure is low,humidity in air; smaller jets.

Long, straight race track - bigger jets, colder sparkplug heatrange.

Short, closed track - smaller jets, hotter sparkplug.

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